1. Field of the Invention
The invention relates to a process for the direct synthesis of methylchlorosilanes using a contact composition comprising strontium.
2. Background Art
Processes for preparing methylchlorosilanes by reaction of silicon with chloromethane in the direct synthesis by the Müller-Rochow method in the presence of suitable catalysts and catalyst combinations are already known. For example, this is described in “Catalyzed Direct Reactions of Silicon”; K. M. Lewis, D. G. Rethwisch; Elsevier 1993.
In the direct synthesis of methylchlorosilanes, metallic silicon is reacted with chloromethane in the presence of various catalysts and, if desired, promoters, with the target product being dimethyldichlorosilane. The mixture of silicon, catalysts and promoters is referred to as a “contact composition.” At present, since over 1,500,000 metric tons per annum of dimethyldichlorosilane are prepared worldwide, small improvements in the production process, for example an increase in the dimethyldichlorosilane selectivity, an increase in the space-time yield specific to dimethyldichlorosilane, or an increase in the specific silicon yield, therefore have a great economic effect.
FR 1037183 and EP 195728 describe the addition of Sr to the contact composition in the direct synthesis. FR 1037183 discloses 1–5% by weight of Sr, while EP 195728 A describes use of Sr in a concentration of from about 0.01 to 2% by weight, with possible catalysts being Cu, CuCl or CuCl2. These high concentrations of Sr have an unfavorable effect on the formation of dimethyldichlorosilane.